Implement SPICE-0009 External Readers (#660)

This adds a new feature, which allows Pkl to read resources and modules from external processes.

Follows the design laid out in SPICE-0009.

Also, this moves most of the messaging API into pkl-core

pkl-codegen-java/src/main/kotlin/org/pkl/codegen/java/CliJavaCodeGenerator.kt

@@ -21,7 +21,7 @@ import org.pkl.commons.cli.CliException
 import org.pkl.commons.createParentDirectories
 import org.pkl.commons.writeString
 import org.pkl.core.ModuleSource
-import org.pkl.core.module.ModuleKeyFactories
+import org.pkl.core.util.Readers
 
 /** API for the Java code generator CLI. */
 class CliJavaCodeGenerator(private val options: CliJavaCodeGeneratorOptions) :
@@ -49,7 +49,8 @@ class CliJavaCodeGenerator(private val options: CliJavaCodeGeneratorOptions) :
         }
       }
     } finally {
-      ModuleKeyFactories.closeQuietly(builder.moduleKeyFactories)
+      Readers.closeQuietly(builder.moduleKeyFactories)
+      Readers.closeQuietly(builder.resourceReaders)
     }
   }
 }